© 2015 Carl Lund, all rights reserved A First Course on Kinetics and Reaction Engineering Class 33

Where We’re Going Part I - Chemical Reactions Part II - Chemical Reaction Kinetics Part III - Chemical Reaction Engineering Part IV - Non-Ideal Reactions and Reactors ‣ A. Alternatives to the Ideal Reactor Models  33. Axial Dispersion Model  D and 3-D Tubular Reactor Models  35. Zoned Reactor Models  36. Segregated Flow Models  37. Overview of Multi-Phase Reactors ‣ B. Coupled Chemical and Physical Kinetics  38. Heterogeneous Catalytic Reactions  39. Gas-Liquid Reactions  40. Gas-Solid Reactions

Axial Dispersion Model The axial dispersion model is a modified form of the plug flow reactor model ‣ it adds a diffusion-like term that accounts for mixing in the axial direction ‣ the quantity D ax is not a diffusion coefficient  it is called the dispersion coefficient  it has one value that is used for all species being modeled  it can be found from correlations of the axial Peclet number (or its inverse, the dispersion number) as a function of the Reynolds number experimentally, for example using stimulus and response measurements involving a tracer (as is done when measuring the age function In commercial scale packed bed reactors, including axial dispersion doesn’t affect conversion as long as the reactor length is greater than ca. 50 catalyst particle diameters ‣ The axial dispersion model can still be used to empirically model a reactor ‣ When D ax = 0, it is a PFR; as D ax becomes very large, it approaches a CSTR

Boundary Conditions and Numerical Solution The axial dispersion model is a second order differential equation ‣ There are several ways the boundary conditions could be formulated ‣ Danckwerts boundary conditions are very common First Danckwerts boundary condition recognizes that as soon as the feed enters the reactor, it will be diluted by axial mixing ‣ Much like what happens to feed entering a CSTR ‣ It requires that the flow of A upstream of the reactor entrance (where no mixing has occurred) must be equal to the net flow of A (due to convection and mixing) at the inlet to the reactor Second Danckwerts boundary condition simply requires that the concentration stops changing at the point where the flow leaves the reactor. The axial dispersion model involves mixed boundary value ordinary differential equations ‣ Numerical solution requires different methods than are used for initial value problems ‣ Discussed in Supplemental Unit S6

Questions?

Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1. The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1. Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1. A → B(1) Read through the problem statement. Each time you encounter a quantity, write it down and equate it to the appropriate variable. When you have completed doing so, if there are any additional constant quantities that you know will be needed and that can be calculated from the values you found, write the equations needed for doing so. Activity 33.1

Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1. The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1. Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1. A → B(1) Generate the mole balance design equations needed to model the axially- dispersed tubular reactor by simplification of the general design equations found in Unit 33 or on the AFCoKaRE Exam Handout.

Where We’re Going Part I - Chemical Reactions Part II - Chemical Reaction Kinetics Part III - Chemical Reaction Engineering Part IV - Non-Ideal Reactions and Reactors ‣ A. Alternatives to the Ideal Reactor Models  33. Axial Dispersion Model  D and 3-D Tubular Reactor Models  35. Zoned Reactor Models  36. Segregated Flow Models  37. Overview of Multi-Phase Reactors